This invention relates to a method of synthesizing a new class of materials by varying the composition throughout the bulk to attain characteristics tailormade for desired applications. The compositions and configurations of the first few coordination spheres of the constituents of a material are controlled to distribute a variety of local environments throughout the material. These synthetic materials are free from the constraints of crystalline symmetries and therefore can yield new types of nonequilibrium disordered structures of varying complexity. The invention enables the production of improved materials which have a wide range of applications in substantially all fields of utilization, for example, in photoresponsive applications, such as solar cells; in superconductivity; in cataylsis; in thermoelectricity; in magnetism; as well as in the development of entirely new materials having properties which can make possible entirely new applications.
Numerous attempts to construct both natural and new crystalline analogue materials have been made with the aim of extending the range of material properties heretofore limited by the availability of natural crystalline materials. One such attempt is compositional modulation by molecular beam epitaxy (MBE) deposition on single crystal substrates. For example, in Dingle et al., U.S. Pat. No. 4,261,771, the fabrication of monolayer semiconductors by one MBE technique is described. These modulated prior art structures are typically called "superlattices." Superlattices are developed on the concept of layers of materials forming a one-dimensional periodic potential by a periodic variation of alloy composition or of impurity density. Typically, the largest period in these superlattices is on the order of a few hundred Angstroms; however, monatomic layered structures have also been constructed. The superlattices can be characterized by the format of several layers of A (such as GaAs) followed by several layers of B (such as AlAs), in a repetitive manner; formed on a single crystal substrate. The desired superlattice is a single crystal synthetic material with good crystalline quality and long range order. The conventional superlattice concepts have been utilized for special electronic and optical effects.
In addition to superlattices, Dingle discloses quasi-superlattices and non-superlattice structures. The former are comprised of epitaxially grown islands of a foreign material in an otherwise homogeneous layered background material. Non-superlattice structures are an extension of quasi-superlattice materials in that the islands are grown into columns extending vertically through the homogeneous layered background material. These superlattice type structures suffer from the same problems that plague homogeneous crystalline materials. There is very little variation possible in the range of constituents and in the type of superlattices constructed, because of the requirements that the spacing between the layers be approximately the same as that of the equilibrium crystalline constituents. These superlattices are restricted to a very small number of relatively low melting point crystalline materials and the growth rates are constrained by the MBE technique.
In addition to the MBE type of superlattice construction techniques, other researchers have developed layered synthetic microstructures utilizing different forms of vapor deposition, including diode and magnetron sputtering and standard multisource evaporation. The layer dimensions are controlled by shutters or by moving the substrates relative to the material sources or controlling reactive gas partial pressure or with combinations of shutters and relative motion. The materials reported have been formed from crystalline layers, noncrystalline layers and mixtures thereof; however, each of the efforts so far reported is directed at the synthesis of superlattice-type structures by precisely reproducing the deposition conditions on a periodically reoccurring basis. These materials can be thought of as synthetic crystals or crystal analogues in which it is crucial that the long range periodicity, repetition of a particular combination of layers or grading or layer spacing be maintained. These structures are both structurally and chemically homogeneous in the x-y plane, but are periodic in the third (z) direction. These construction approaches can utilize a greater variety of materials; but as previously reported, they have not produced as great a variety of structures as have been demonstrated by MBE techniques.
The previous works of the undersigned, as described for example in U.S. Pat. Nos. 4,177,473; 4,177,474 and 4,342,044, describe forming various types of matrices and dispersing nonequilibrium configurations therein. The principles of layering and compositional modulation in the prior work is dependent upon the amounts of material distributed on an atomic and molecular scale throughout the material. For example, as described in the prior work, very small amounts of material were added to a matrix to increase the bulk resistance properties of the resulting material. The addition of larger amounts of material to the matrix decreases the bulk resistance properties. The principles are utilized to modify a range of materials from metallic to dielectric materials. These techniques provide a distribution of material configurations on an atomic and molecular scale from microscopic to macroscropic configurations in the matrix material. The distribution of compositional changes of a primarily nonequilibrium nature lead from individual atoms or groups of atoms to layering when sufficient amounts of material are introduced into the matrix. These techniques provide significant control of material properties such as thermal and electrical conductivity and other parameters by introducing atoms and alloys into materials which bond in a manner not previously described.
This invention allows for the first time even more fundamental control of material properties by freeing materials not only from crystalline symmetry, but also from the periodic local order required in previous amorphous and disordered materials. By the principles and methods disclosed herein, spatial and orientational placement of similar or dissimilar atoms or groups of atoms is possible with such increased precision and control of the local configurations to result in qualitatively new phenomena. The atoms need not be restricted to "d band" and "f band" atoms, but can be any atom in which the controlled aspects of the interaction with the local environment plays a significant role physically, electrically or chemically so as to affect the physical properties and hence the functions of the materials. This results in means of synthesizing new materials which are disordered is several different senses simultaneously. Such structures can be referred to as "multi-disordered".